In current cochlear implant (CI) pulsatile coding strategies, the acoustic signal is usually decomposed into a set of band pass signals. The number of band pass signals is often equal to the number of electrodes contacts in the system. Thus relatively broad band passes have to be used to cover the entire acoustic frequency range transmitted by the system. With current CI systems, users already enjoy good speech understanding in silent and tolerably noisy conditions, but musical perception remains relatively unsatisfying. That may be because some musical characteristics such as frequency harmonics are not be perceived by current CI users.
For many musical instruments, the fundamental frequency (which particularly describes the pitch perception in the auditory system) is located between 100 Hz and 400 Hz. Transmission of the fundamental frequency and its first order higher harmonics is not yet accomplished in current CI systems. To adequately improve the perception of chords for CI users, the transmission of music harmonics should be advanced so that single harmonics can be represented on the basilar membrane according to their pitch.
In current CI systems, the filter band widths are usually greater than 100 Hz such that more than one harmonic is processed by each filter band. In current fine structure coding strategies, the stimulation timing is derived from the band pass signals, in which case, when multiple harmonics fall within a given filter band, the derived stimulation timing is usually not representative of any of the harmonics but instead depends on the relative amplitudes and the frequency spacing. This means that the stimulation timing in low-to-mid frequency channels is relatively complex rather than simply coding the periodicity of the dominant harmonics; usually one harmonic dominates a filter band. In normal hearing, such a harmonic masks any neighboring harmonics and carries the information that should be transmitted tonotopically and temporally correct.
In psychoacoustic pitch testing both concepts, periodic pitch and tonotopic pitch have been demonstrated to work in CI patients. To be more precise, a gradual shift of stimulation from an apical electrode towards a more basal one at relatively high rates leads to an increase in pitch percept. Nobbe et al. (Acta Oto-Laryngologica, 2007; 127:1266-1272) showed that both, simultaneous or sequential stimulation lead to just noticeable pitch changes of down to one semitone. Similar results can be found if the low stimulation rate of one electrode is increased. Pitch JNDs range down to below one semitone. These results suggest that a combination of both cues could lead to an even improved pitch perception in CI users.
One coding strategy that partially addresses the above is the Fine Structure Processing (FSP) strategy used in the Med-E1 OPUS 1 and OPUS 2 speech processors. The FSP strategy codes very low frequency harmonics, usually the fundamental frequency and the second harmonic, by using a filter bank that ranges down to below the expected fundamental frequencies. The spacing of the lowest frequency bands is such that the harmonics coded are usually resolved, that is, only one harmonic falls into one low frequency filter band. But higher harmonics are not explicitly resolved by this type of signal processing. In addition, the shift of harmonics is mainly coded temporally. A tonotopic shift of the temporal code of fundamental frequency gliding from 100 Hz up is only achieved at around 200 Hz. Explicit mapping of tonotopicity according to channel specific stimulation rates is not possible with this strategy.
The HiRes 120 strategy of Advanced Bionics Corporation uses active current steering and additional spectral bands. The input signal is filtered into a large number of spectral bands and fast Fourier transformation (FFT) algorithms are applied for fine spectral resolution. Hilbert processing derives temporal detail from the signals while the spectral maximum for each electrode pair is determined across all the filter bands. Pulse rate and stimulus location are determined from the estimated frequency of the spectral maximum. A number of spectral bands are assigned to each electrode pair and the spectral bands are delivered to locations along the electrode array by accurately varying the proportion of current delivered simultaneously to adjacent electrodes in each electrode pair. To our knowledge the strategy does not contain any means to correct the mismatch between stimulation rates derived from sub bands and tonotopicity.
Another stimulation concept was described by Kasturi K. & Loizou P., Effect Of Filter Spacing On Melody Recognition: Acoustic And Electric Hearing, Journal of Acoustic Society of America, 122(2), 2007, p. EL 29-EL 34; incorporated herein by reference. A semitone filter bank was used to more accurately analyze melodies. A set of 12 semitone filters was assigned to an equal number of stimulation electrodes. They divided a certain frequency range into a number of channels and analyzed the melody recognition of CI users. They pointed out, that the melodies processed with the semitone filter spacing were preferred over melodies processed by the conventional logarithmic filter spacing.
In U.S. Patent Publication 20080021551 (incorporated herein by reference) a system for optimizing pitch perception in CIs is described where a reference signal is generated and applied to an appropriate electrode on the electrode array. A probe signal having a fixed interval relationship with the reference signal is applied to an appropriate electrode on the electrode array. The location of the probe signal or reference signal is shifted until the two signals match. A frequency map is applied that uses the location at which the probe signal and the location at which the reference signal is applied when the two signals match. The frequency map is used to apply stimulus signals to correct locations within a cochlea as a function of pitch.